Transmission spectroscopy presents one of the most straightforward methods for characterizing the atmospheres of exoplanets. The technique involves analyzing the planetary absorption imprinted on the stellar light that has passed through the planet's atmosphere. This makes hot and expanded atmospheres, such as those of hot gas-giants, as ideal targets. Coupled with high-resolution spectroscopy, it is possible to distinguish hundreds to thousands of planetary spectral lines, and combining them together via cross-correlation technique will enhance the planetary atmosphere signal, enabling the robust detection of individual chemical species. Here we report the detection of water vapor (H2O) at >5-sigma in the near-infrared transmission spectrum of the hot-Saturn HD 149026b, using archival data from the CARMENES spectrograph (R~80,400). With a likelihood approach, we were able to constrain the planet's semi-amplitude radial velocity as well as its systemic velocity. Interestingly, the latter was significantly red-shifted relative to the expected value. This might be due to a number of factors, such as errors in the transit mid-point or even a signature of atmospheric dynamics from e.g., high-altitude winds. Searches for HCN and CH4 were unsuccessful, and subsequent model injection tests indicated that this was likely due to the insensitiveness of the data to these species (due to the relatively low S/N), at least for the assumed atmospheric composition. Finally, the detection of H2O in this high-density planet, along with future constraints on its abundance, will provide valuable insights into the planet's formation history.